宏：来自 char 字面量的字符串字面量

Question

Is there a way in C to create a string literal from a character literal, using a macro?

for example I have

'a'

and I want to create the string literal

"a"

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To clarify the question:

#define A 'a'

write(fd, "x=" CHAR2STRING(A) "\n", 4);

My question is how to define the macro CHAR2STRING

Answer 1

–Summary of the comments to the question–

This seems impossible to achieve. As an alternative, the string literal could be defined and a STRING2CHAR macro be written instead:

#define A "a"
#define STRING2CHAR(s) (*(s))
write(fd, "x=" A "\n", 4);
putchar(STRING2CHAR(A));

or

#define A a
#define XSTR(s) #s
#define SYM2CHAR(sym) (*XSTR(sym))
#define SYM2STRING(sym) XSTR(sym)

The expression *"a" isn't a compile-time constant (so eg it cannot be used as an initializer for an object with non-automatic storage duration, a non-VLA array length, a case label, or a bit-field width), though compilers should be able to evaluate it at compile-time (tested with Gcc and Clang).

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^{Suggested by M Oehm and Matt McNabb .}

Answer 2

Although I'm quite late to the party, here's a solution I came up with for ASCII (7 bits) characters:

#define CHAR2STR(c) ((char*[]){ \
    "\0", "\1", "\2", "\3", "\4", "\5", "\6", "\a", \
    "\b", "\t", "\n", "\v", "\f", "\r", "\16", "\17", \
    "\20", "\21", "\22", "\23", "\24", "\25", "\26", "\27", \
    "\30", "\31", "\32", "\33", "\34", "\35", "\36", "\37", \
    " ", "!", "\"", "#", "$", "%", "&", "'", \
    "(", ")", "*", "+", ",", "-", ".", "/", \
    "0", "1", "2", "3", "4", "5", "6", "7", \
    "8", "9", ":", ";", "<", "=", ">", "?", \
    "@", "A", "B", "C", "D", "E", "F", "G", \
    "H", "I", "J", "K", "L", "M", "N", "O", \
    "P", "Q", "R", "S", "T", "U", "V", "W", \
    "X", "Y", "Z", "[", "\\", "]", "^", "_", \
    "`", "a", "b", "c", "d", "e", "f", "g", \
    "h", "i", "j", "k", "l", "m", "n", "o", \
    "p", "q", "r", "s", "t", "u", "v", "w", \
    "x", "y", "z", "{", "|", "}", "~", "\177", \
})[c]

This can be extended to the full ASCII table with "\180", "\181" ... (all numbers are octals, so no \178 !), but I don't really think it would be useful, considering it's a MACRO. The trick works thanks to an anonymous string array ("table") initialized in place, which should be optimized by the compiler automatically (if the character is known).

Answer 3

Not very elegant, but it works:

#define STRING_ME(tgt, ch) tgt[0]=ch;tgt[1]='\0'

Assumes tgt has space for 2 chars. Perhaps you could give an example what you want it to look like?

Answer 4

You could do

#define A 'a'
#define X(macro) #macro
#define CHAR2STRING(macro) X(macro)

printf("%s\n", CHAR2STRING(A));

you will get 'a' instead of a, but maybe thats ok for you.